The project's objective is the development and production of an optical fiber-based oxygen microsensor which is sensitive in vitro to changes in oxygen concentration within cell cultures. The value of this sensor system is in bioassays based on measuring oxygen metabolism in cell cultures, cell toxicity studies, drug testing and environment quality testing. The aims of Phase I are to test the feasibility of the sensor design for accurately determining the rate and quality of cells' oxygen respiration during normal growth and under various toxic or stimulatory conditions. The sensor is fabricated by affixing a phosphorescent indicator compound to the distal tip of a fiber optic. Excitation light projected through the fiber excites the phosphor which emits a wavelength shifted long-lifetime decay signal returned to a detector. The phosphor's emission is affected by the presence of oxygen which quenches the long-lifetime luminescence. It is thereby practical to optically track oxygen levels in the microenvironment of cells at the sensor tip based on the lifetime of the phosphorescence. The technology developed will yield a family of versatile bioassay products with applications that span research in cell metabolism, toxicology, cancer biology, development of therapeutics, and diagnostics to clinical medicine.